A captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer. The process usually involves a computer asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the captcha, any user entering a correct solution is presumed to be human. A common type of captcha requires that the user type letters or digits from a distorted image that appears on the screen.